1. Field of the Invention
The present invention relates to a resin roll for calendering a magnetic recording medium and to a manufacturing method therefore.
2. Description of the Background Art
Generally, it is a wide spread practice in the manufacturing of a magnetic recording medium to apply a magnetic layer on a base film and to perform calendering thereafter.
Generally, in the process of manufacturing a magnetic recording medium, the magnetic recording medium is calendered by being passed between a mirror-surfaced metal roll and an elastic roll such as a resin roll disposed in opposing relationship thereto. A high nip pressure is applied to the recording medium to eliminate voids in the magnetic layer, to make the surface of the magnetic recording medium smooth and to increase the density of the magnetic layer. In this case, the magnetic layer side of the magnetic recording medium is brought into contact with the metal roll.
The signal density of the magnetic recording medium has been significantly improved recently. In order to obtain a highly dense magnetic recording medium, it is necessary to apply a magnetic coating filled with magnetic powder to a high density on the base film so as to increase the magnetic flux density. However, if the amount of the magnetic powder is increased, it becomes difficult to eliminate voids in the magnetic layer generated when the solvent is dried, and also it has been difficult to obtain sufficient surface smoothness of the magnetic layer under the conventional conditions for calendering, because hardness of the magnetic coating could have been increased.
Accordingly, in order to make smooth the surface of the above described magnetic layer filled with magnetic powder to a high density and to completely eliminate voids, it is necessary to apply higher temperature and higher nip pressure in the calendering step.
Accordingly, there has been a demand for a resin roll for calendering which can be used at higher temperature and which can provide greater pressure.
The following characteristics are required of the resin roll for calendering.
(1) A roll with satisfactory high surface smoothness. PA1 (2) Hardness, especially high surface hardness. PA1 (3) Heat resistance, especially heat resistance to render the roll less likely to deform due to thermal expansion or melting that would result from autogenous heat. PA1 (4) Compression strength to withstand the high nip pressure applied by the metal roll and to remain free of cracking or breaking. PA1 (5) A roll free of pin holes. PA1 N: nip width PA1 R.sub.1 : radius of first roll PA1 R.sub.2 : radius of second roll PA1 P: load power unit length PA1 .nu..sub.1 : Poisson's ratio of the first roll PA1 .nu..sub.2 : Poisson's ratio of the second roll PA1 E.sub.1 : elastic modulus of the first roll PA1 E.sub.2 : elastic modulus of the second roll PA1 n: the ratio of the circumference of a circle to its diameter.
However, in the conventional resin roll for calendering a magnetic recording medium, the roll properties are limited, so that the nip width, that is, the nip area tends to be unavoidably large. Therefore, in terms of applied pressure, that is, pressure per unit area acting on the nip surface, satisfactory pressure per unit area to meet the recent demand for high density magnetic recording medium could not be obtained. Furthermore, in order to improve efficiency in processing, when heating is performed at a higher temperature or when the press load is increased to obtain larger pressure per unit area, the nip width, that is, the nip area is further enlarged, making it difficult to obtain the effective pressure per unit area at the nip surface.